A heat pump hot water heating system for heating or supplying hot water utilizing a heat pump cycle includes a hot water circulation circuit for heating, in which hot water heated with a refrigerant of the heat pump cycle via a heat exchanger is supplied to an indoor radiator that heats the room or to a hot water tank, and then the hot water which has been used is returned to the heat exchanger of the heat pump cycle. To control the hot water to be supplied, the hot water circulation circuit for heating includes a supply temperature sensor that detects the supply temperature of the hot water to be supplied.
Conventional heat pump hot water heating systems generally employ the supply temperature control because of quick response and ease of controlling, to cope with fluctuation in heat pump capacity (compressor operation frequency) of the heat pump cycle. However, the heating system is operated with the minimum supply capacity through control of the compressor operation frequency on the heat source side of the heat pump cycle, for example when the efficiency of the indoor radiator on the load side of the hot water heating system is degraded, or when the required air-conditioning and hot water supply load is small such as in a transitional period between seasons. In such a case, when the heat amount supplied by the operation with the minimum supply capacity is larger than the heat transfer amount on the side of the air-conditioning and hot water supply load, the supply temperature of the hot water circulation circuit for heating exceeds the target supply temperature, and therefore the operation of the compressor is set to an on/off cycle operation owing to the quick response.
When the compressor enters the on/off cycle operation mode, not only the efficiency as a heat pump hot water heating system is degraded, but also the life span of the parts of the refrigerant circuit including the compressor and that of electric circuit parts may be shortened, owing to pressure fluctuation of the refrigerant circuit and repeated switching actions of relay contacts of the electric circuit, incurred by frequent on/off switching of the compressor.
An ideal measure for the mentioned drawback is reducing the operation frequency of the compressor so as make the heat amount based on the minimum supply capacity of the heat pump heat source apparatus equal to the heat transfer amount (in the case of hot water heating operation) or heat removal amount (in the case of cold water cooling operation) on the side of the air-conditioning and hot water supply load, regardless of how small the heat transfer amount or the heat removal amount is. However, a lower limit of the operation frequency is set in the compressor from the viewpoint of reliability thereof, and therefore the mentioned measure is still insufficient.
Accordingly, some of the conventional heat pump hot water heating systems are configured to lower the return temperature by reducing the rotation speed of the heating circulation pump to thereby reduce the flow speed of the circulating water when a decrease in heating load is detected, and then activate a mixing valve to open a bypass flow path so that a part of the discharged hot water joins the heating hot water, so as to lower the supply temperature (for example, see Patent Literature 1).
In addition, some heat pump hot water heating apparatuses are configured to gradually increase the rotation speed of the heating circulation pump after activating the heat pump until the heat pump cycle is stabilized, to control the rotation speed of the circulation pump so that the supply temperature reaches the target temperature, and then to control the rotation speed of the circulation pump so that the return temperature agrees with the target temperature (for example, see Patent Literature 2).